Add failing tests for Worksheet class bugs

Tests cover: fields property returning method object instead of list,
missing None checks on rows/cols/datasources elements causing crashes,
and incomplete collection of datasource-dependencies columns.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacalata

test/assets/worksheet_no_datasources.twb

@@ -0,0 +1,20 @@
+<?xml version='1.0' encoding='utf-8' ?>
+<workbook source-build='9.3.3 (9300.16.0603.2240)' source-platform='win' version='9.3' xmlns:user='http://www.tableausoftware.com/xml/user'>
+  <datasources>
+    <datasource caption='TestData' inline='true' name='federated.abc123' version='9.3'>
+      <connection class='federated'>
+        <named-connections />
+      </connection>
+    </datasource>
+  </datasources>
+  <worksheets>
+    <worksheet name='Sheet 1'>
+      <table>
+        <view>
+        </view>
+        <rows></rows>
+        <cols></cols>
+      </table>
+    </worksheet>
+  </worksheets>
+</workbook>

test/assets/worksheet_no_shelves.twb

@@ -0,0 +1,21 @@
+<?xml version='1.0' encoding='utf-8' ?>
+<workbook source-build='9.3.3 (9300.16.0603.2240)' source-platform='win' version='9.3' xmlns:user='http://www.tableausoftware.com/xml/user'>
+  <datasources>
+    <datasource caption='TestData' inline='true' name='federated.abc123' version='9.3'>
+      <connection class='federated'>
+        <named-connections />
+      </connection>
+    </datasource>
+  </datasources>
+  <worksheets>
+    <worksheet name='Sheet 1'>
+      <table>
+        <view>
+          <datasources>
+            <datasource caption='TestData' name='federated.abc123' />
+          </datasources>
+        </view>
+      </table>
+    </worksheet>
+  </worksheets>
+</workbook>

test/test_workbook.py

@@ -22,6 +22,16 @@
     'filtering.twb'
 )
 
+WORKSHEET_NO_DATASOURCES_FILE = os.path.join(
+    TEST_ASSET_DIR,
+    'worksheet_no_datasources.twb'
+)
+
+WORKSHEET_NO_SHELVES_FILE = os.path.join(
+    TEST_ASSET_DIR,
+    'worksheet_no_shelves.twb'
+)
+
 
 class EphemeralFields(unittest.TestCase):
     def test_ephemeral_fields_do_not_cause_errors(self):
@@ -56,4 +66,72 @@ def test_worksheets_setup(self):
         self.assertEqual(len(wb.worksheet_items), 2)
         worksheet_names = [ws.name for ws in wb.worksheet_items]
         worksheet_names.sort()
-        self.assertEqual(worksheet_names[0], 'Sheet 1')
+        self.assertEqual(worksheet_names[0], 'Sheet 1')
+
+    def test_worksheet_fields_returns_list_not_method(self):
+        # fields property was returning the method object instead of self._fields
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        fields = ws.fields
+        self.assertIsInstance(fields, list, "fields should return a list, not a method object")
+
+    def test_worksheet_fields_are_field_objects(self):
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        from tableaudocumentapi import Field
+        for f in ws.fields:
+            self.assertIsInstance(f, Field)
+
+    def test_worksheet_datasources_not_empty(self):
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        self.assertGreater(len(ws.datasources), 0)
+
+    def test_worksheet_rows_returns_list(self):
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        # Sheet 1 has two fields on rows shelf
+        self.assertIsNotNone(ws.rows)
+        self.assertIsInstance(ws.rows, list)
+        self.assertEqual(len(ws.rows), 2)
+
+    def test_worksheet_cols_empty_shelf_returns_none_or_list(self):
+        # Sheet 1 has an empty <cols /> element - should not crash
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        result = ws.cols
+        self.assertTrue(result is None or isinstance(result, list))
+
+    def test_worksheet_rows_field_names(self):
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        # Sheet 1 rows: Calculation_88946136969252864 (caption: SHOW) and "Burst Out Set list"
+        # _ds_fields_to_items resolves via datasource.fields, so we get Field objects or raw strings
+        from tableaudocumentapi import Field
+        names = [f.caption if isinstance(f, Field) else f for f in ws.rows]
+        self.assertIn('SHOW', names)  # Calculation resolved to its caption
+
+    def test_all_datasource_dependencies_collected(self):
+        # return inside loop meant only first datasource-dependencies block was processed
+        wb = Workbook(DASHBOARDS_FILE)
+        ws = wb.worksheet_items[0]
+        # Sheet 1 has 3 columns in its datasource-dependencies block
+        self.assertEqual(len(ws.fields), 3)
+
+    def test_worksheet_with_no_datasources_element_does_not_crash(self):
+        # _prepare_datasources iterates the result of find(), which is None if element absent
+        wb = Workbook(WORKSHEET_NO_DATASOURCES_FILE)
+        ws = wb.worksheet_items[0]
+        self.assertEqual(ws.datasources, [])
+
+    def test_worksheet_with_no_rows_element_does_not_crash(self):
+        # _prepare_rows calls .text on find() result without None check
+        wb = Workbook(WORKSHEET_NO_SHELVES_FILE)
+        ws = wb.worksheet_items[0]
+        self.assertIsNone(ws.rows)
+
+    def test_worksheet_with_no_cols_element_does_not_crash(self):
+        # _prepare_cols calls .text on find() result without None check
+        wb = Workbook(WORKSHEET_NO_SHELVES_FILE)
+        ws = wb.worksheet_items[0]
+        self.assertIsNone(ws.cols)